Various RF (radio frequency) modulation schemes are known. 2-level FM (frequency modulation) and 4-level FM are commonly used. 2-level FM ("2-level") has good sensitivity performance at the receiver, but poor multipath distortion performance. 4-level FM ("4-level"), while having poorer sensitivity performance than 2-level, has better multipath distortion performance for simulcast systems because 4-level uses a clock at half the rate of the 2-level clock and therefore can tolerate more inter-symbol interference.
The most common communication systems are not simulcast systems. Hence most systems are 2-level systems in order to benefit from better sensitivity performance. However, simulcast system use is increasing as is the size of simulcast systems.
If a large user group, having 2-level communication units, such as radios, base stations and the like, in its system, decided to switch to a simulcast system to attain the benefits of a simulcast system, poor performance would be realized because of distortion. The radios are unable to modulate or demodulate 4-level transmissions. To replace all the radios with 4-level radios would be expensive, and 4-level still suffers from poor receiver sensitivity.
A possible solution is to have a system which combines 2-level and 4-level in order to minimize disadvantages of each. Using 2-level modulation inbound to the infrastructure as well as in talk-around and 4-level outbound from the infrastructure (simulcast transmissions) would minimize distortion while maximizing sensitivity. If a retrofit of the modulation/demodulation circuits in existing radios were done to minimize costs, a system with 2-level and 4-level would be difficult because the additional 4-level circuitry and the 2-level to 4-level interface circuitry would take up at least twice as much space as the previous 2-level circuitry, and it is unlikely that the circuitry would fit in the radio.
Furthermore, a big difference between 4-level and 2-level systems is repetitive pattern recognition. Many 2-level systems use a binary 10101010 pattern for various signal functions, such as end-of-message (EOM). This 10101010 pattern has good dc signal balance characteristics for 2-level modulation. However, if this same 10101010 pattern were modulated into a 4-level signal, the result would be a fiat signal, which has no dc signal balance. Hence, a different pattern must be chosen for 4-level EOM detection. The problem with changing this pattern is that existing voice and data processing circuits which use this EOM signal are not capable of detecting a new pattern, and making new circuits for this detector is a highly costly endeavor.
Accordingly, what is needed is an apparatus which can inexpensively convert a normal 2-level radio into a radio capable of receiving both 2-level and 4-level signals while providing pattern recognition without changing existing voice and data circuits.